Patent GB-1414164-A (GB)
Title: Correlator and control system for vehicular collision avoidance
IPC: G01S13/933
Filed: 1973-01-18
Abstract: 1414164 Radio navigation RCA CORPORATION 13 April 1973 [27 June 1972] 30036/72 Heading H4D The invention is conerned with a digital correlator particularly as used in an airborne anti-collision navigation system of the Secant type as described in Specifications U.K. 1,278,451 and U.S.A. 3,803,608. In such a system each aircraft taking part, e.g. 10, Fig. 1, transmits a series of pulses spaced at nominal 1 millisecond intervals the pulses being either a P-type or a a Q-type with the selection of the type being on a random basis utilizing the output of a random noise generator. The other aircraft reply-to this interrogation by a series of like spaced and type pulses 22, 24, 26, 28 whereby, by correlation and using the very high correlation factor produced by the random pulse spacing and pulse type, an accurate separating out of the pulses from the various responding aircraft can be made and the range and potential collision danger of said aircraft determined. The random noise source, 60, Fig. 6a produces an output 62 normally fluctuating between two logic levels 1 and 0. To determine whether the transmitted pulse should be of the P-type or the Q-type a flip-flop 64 clocked at 1 kilohertz samples the noise at 1 millisecond intervals and produces an upper level 106 which enables a Q-type determining AND gate 98 for 1 millisecond or a lower level 108 which via inverter 112 enables a P-type determining AND gate 100 for 1 millisecond. To determine how much the repetition interval of the pulses shall differ from 1 millisecond, i.e. how much the jitter shall be, eight consecutive samples of the noise are stored in a register 70 and the cumulative output on 72 used to preload a counter 78 which is fed with the output of a 1 megahertz clock 82 to give a transmission determining output via a decoder 86 which would occur once every millisecond but for the preloading. Such output via a one shot 90 produces a time zero TO pulse which on line 102 is fed to the gates 98 and 100 passed through that one of the gates which has been enabled by the output of the flip-flop 64 to become the P- or the Q-type probe pulse from transmitter 52. The output of the flip-flop 64 is also fed to correlating AND gates 116 118, 120, 122 as the reference signal for correlation of any received responses. Any responses are received by receiver 54 and separated out into the Q-types and the P-types. The Q-type pulses are fed to AND gates 118 and 122 and the P-type pulses to AND gates 116 and 120. If responses from two aircraft are received simultaneously an AND gate 130 produces an output which via an inverter 122 disables the AND gates 116 to 122. AND gates 116 and 118 produce outputs when no correlation is determined between the reference signal and the received responses whilst AND gates 120 and 122 produce an output when such correlation is detected. The outputs of the AND gate correlators are integrated in 15 up-down counter-6in-accumulators 124 to 128. The outputs of all the AND gates are collectively fed to the accumulators via OR gate 144 whilst those from the like correlation AND gates 120, 122 are fed via an up down control flip-flop 150 to determine whether the used accumulator should count up or count down. Each time zero pulse is fed to a range band control circuit 91 to cause it to send a pulse to a shift register 94 which then produces a series of contiguous range band pulse T1 to T16 1 microsecond in duration, which sequentially enable the accumulators 124 to 128. The accumulators therefore act as range bins. When the count in any particular range bin accumulator has reached a threshold indicating that a target response has been detected at the range of said range bin a comparator 162, 166 produces an output, which via target address encoder 182 causes a tracker 56, e.g. as described in U.S.A. Specification 3,803,604 to analyse the response as to wether it is from a truly dangerous aircraft. If none of the comparators produce an output after 0À1 second the range band control 91 causes its next pulse on line 160 to occur a time after the time zero pulse such that the new set of range band bin pulses are generated whereby the up down counter bin accumulators are sequentially enabled for ranges greater than the set of ranges initially used. If however one or more of the comparators produce outputs the initial set of range band bin pulses are used until the tracker 56 has analysed all of the responses.